Location of radio transmitters is a technology that has been used for many years and in many applications. In general, radio direction finding is classified into many subclasses using a variety of direction finding tools such as antenna loops and antenna arrays. For radio location, the use of mobile transmitters and a plurality of fixed base station receivers is generally known in the art where the fixed receivers locate the source of the radio signal by triangulation of the radio signals. In the alternative, a plurality of fixed base stations may transmit radio signals and a mobile receiver may determine its own position.
Triangulation is a technique where the fixed base station receivers each determine the approximate direction of the radio signal from the transmitter. The approximate direction lines are then drawn on a map where the lines will cross. Since the direction lines are approximate, and typically three fixed base station receivers are used, the intersecting area between the three lines is usually shaped like a triangle since the lines rarely meet at a single point. The transmitter is then located within the triangle. Triangulation of radio signals is fraught with uncertainty. Propagation interference, signal drop out, insufficient transmitter power, ambiguity in beating measurements received from behind the receiving antennas and multi-path reflections are common sources of errors and uncertainty in radio direction finding and location.
Vehicle location systems are known in the art which use a variety of direction finding and location techniques. One example of a vehicle location system is U.S. Pat. No. 4,740,792 to Sagey et al. which describes a mobile transmitter and a plurality of satellite receivers. A central site determines the location of the mobile transmitters by calculating the difference in the arrival time of the spread spectrum transmitter signals at the satellites. This system may be used, for example, for the anti-theft location of stolen vehicles or the location individuals carrying the transmitters.
In the alternative, a plurality of fixed base stations may transmit radio signals and a mobile receiver may determine its own position as for example in Loran-C or Global Positioning Satellite Systems. One of these types of systems is exemplified in U.S. Pat. No. 4,665,404 to Christy et al. which describes the use of spread spectrum radio transmitters at fixed locations and mobile receivers. The receivers locate their own position by accurately measuring the time at which the radio signals are received to determine the range from the transmitters.
There is a need in the art for a ground-based vehicle or mobile transmitter location system which locates a large number of small, low-power transmitters and which operate without confusion or collision. There is a further need for a ground-based mobile transmitter location system which is immune from multi-path disruption of the radio signals in an urban or irregular terrain environment. There is a further need for a ground-based mobile transmitter location system which is capable of carrying messages or identification signals simultaneous with the location of the transmitter.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a low-power, spread spectrum vehicle or personal locator system which operates effectively in moderate to severe multi-path conditions without collisions of simultaneous messages from a plurality of transmitters.